How to understand central heating system

8/6/2019 How to understand central heating system

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HOW-TO 1

Wise homeowners have their domestic boilers and heating systems checked regularly by a competent

heating engineer. Nonetheless, it is sensible to be familiar with the layout and terminology of the system.

This will facilitate better communication with an engineer in an emergency.

Please refer to How-to Understand your Hot and Cold Water System.

HOW-TOHOW-TO UNDERSTAND YOUR CENTRAL HEATING SYSTEM

Adjustable spanner, 250mm

Adjustable spanner, 100mm

Square nose pliers, 150mm

Footprint wrench, 225mm

Pozidrive Nos 1 and 2 screwdrivers

Flat-bladed screwdrivers, with 3mm, 6mm and10mm tips

Combination stop valve key, to fit fiin and flinsquare heads and fiin and flin tee handles

Radiator bleed key

CENTRAL HEATING

In old systems, central heating is operated on the same gravity principle described in Direct Hot Water in

How-to Understand Your Hot and Cold Water System. Not only does it require large bore pipes to handle

the flow of water, but these pipes tend to fur up with lime scale. The system is very inefficient.

Pumped open systemWater is heated by a boiler and pumped through small-bore pipes 22mm and 15mm to efficient radiators.

Cold water is reheated by the boiler as it is pumped round. Any minimal loss of water due to evaporation is

topped up from a feed and expansion tank. A pipe from a high point in the system allows the water to expand and

contract. If the water overheats, steam will escape into the tank.

TOOLS

The following tools are useful in all emergencies and are also handy for maintenance.


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HOW-TO 2

The boiler is also used to heat domestic hot water via a heating coil in a cylinder which may be gravity fed (F1).In almost all modern systems, both the primary and central heating water are pumped (F2).

F1

F2


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HOW-TO 3

Sealed pressurised systems

These have no feed and expansion tank, but they do have a closed expansion vessel close to the boiler (F3).Inside the vessel is a flexible membrane sealing off a quantity of air or nitrogen that has been pumped in to a

predetermined pressure. As water expands into the vessel, the water pressure increases. As the expansion vessel

is smaller than the tank, the design can be more flexible. It is efficient and easy to install. Only convector radiators

should be used with this system, because of a higher water temperature.

F3


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HOW-TO 4

BOILERS

There are four fuels available for heating boilers solid fuel (coal or wood), oil, mains gas and liquefied petroleum

gas (LPG). All have advantages and disadvantages.

SAFETY

It is not recommended that DIY householders install or maintain gas appliances. Do not tamper with or attempt to

alter gas appliances or flues without taking expert advice. By law, professional installers must be registered with

the Council for Registered Gas Installers (CORGI). If you do suspect a gas or water leak, shut down the boiler

and contact your gas supplier or a CORGI professional immediately. For a gas leak, shut off the valve next to the

gas meter and ventilate the house.

C O NV E NT I O NA L B O I LE R S

These are floor standing and, depending upon design, may be fuelled by gas, oil or solid fuel, which heats water

via a heat exchanger. General reliability and long life offset their inefficiency and higher running costs. They

require a chimney or must abut an outside wall for a balanced flue.

Solid fuel boilers go out unless topped up with fuel and are maintained on a daily basis. There are two types:

hopper feed, which usually requires filling once a day (F4), and hand feed, which needs regular filling (F5).

Gas- and oil-fired boilers are easily controlled and need very little maintenance.

F4

F5


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HOW-TO 5

O I L F I RE D B OI L ER S

These are cheaper to run than gas-fired ones, but the maintenance costs are higher. Oil is supplied from a storage

tank, pumped or gravity fed. In order to burn, oil must either be turned into fine droplets (atomised) (F6) or turnedinto gas (vaporised) (F7).

Conventional, combination and condensing boilers can all be oil fired.

COMBINAT ION (COMBI ) BOILERSThese may be gas- or oil-fired. They are usually wall mounted and are neat and compact. Being a pressurized

system, the pressure vessel is built into the boiler, as are the circulating pump, pressure gauge, air vent and safety

and pressure relief valves (F8). If LPG is used, it is supplied from a large permanent storage cylinder and needsspecial gas jets.

Central heating water is in an enclosed pressurized system. Water for domestic hot taps is fed from the rising

main, through the boiler and directly to the hot taps, as required. No storage tanks are needed. These systems

are very suitable for flats and small houses, when water pressures are reliable. In a large house where several

people may require hot water simultaneously, the system is not recommended.

CONDENSING BOILERS

These are very efficient, using less fuel than other boilers for the same heat output. Water returning from the

heating system extracts the heat from the exhaust gases, which are wasted in other boilers.

This system is available for fully-pumped open systems, pressurized systems or for combination boilers. All types

require a fan-assisted flue.

F6

F7

F8


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HOW-TO 6

BACK BOILERS

These may be fuelled by gas (F9), oil or solid fuel. All require an open flue (chimney), while oil and gas boilersrequire a lined flue.

Coal-fired boilers (F10) only work when the front fire is lit. An electric immersion heater is required during thesummer.

EXPLANATION OF COMPONENTS IN DIAGRAMS

Overflow warning pipe (F1 and F2). This usually runs from the tank or cistern to the outside of the building. Insome cases, a WC cistern overflow pipe may discharge inside the property in such a way that the water may be

seen. The diameter of the overflow pipe is always larger than that of the supply pipe. This ensures that water will

not overflow the edges of the tank or cistern.

Drain taps may be straight or bent (F19). They may be combined with a stop-valve and all allow a system to bedrained down. A hose is attached to the nozzle or spigot and the tap opened with a small spanner or special key.

A Pressure gauge is a dial indicating the running pressure of a system in bar or foot-pounds.

The Circulating pump (F26) pumps water round the system.

F9

F10

F19

F26


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HOW-TO 7

F L O AT VA LV E S

Referred to in the past as ball-cocks, these are fitted to water storage tanks and WC cisterns to control water levels.

As the float rises with the water level, it shuts off the valve at a predetermined point. The three most commonly

used float valves are:

Portsmouth (F11). As the water level drops, the arm moves a piston away from a nozzle, allowing water to flow.

Diaphragm or Garston (also known as BRE or BRS) (F12). The water inlet nozzle is closed by the action of thefloat arm pushing a plunger onto a rubber diaphragm. The filling action of this valve is gentler and quieter than the

Portsmouth valve.

Servo-diaphragm or Torbeck. This operates in a more sophisticated way than the Garston valve. It discharges

water into the cistern via a collapsible plastic valve. The water delivery is fast and very quiet, while the arm and

float are quite small by comparison. This valve is only for use in WC cisterns.

Less common valves are the Croydon valve (F13), now obsolete, and the Equilibrium valve (Portsmouth pattern).Similar in pattern to the Portsmouth valve, it is designed to overcome supply problems where the water pressure

fluctuates.

F11

F12 F13


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HOW-TO 8

C O NT R OL VA LV E S

Gate or Fullway valves (F14) are used to regulate the flow of water and when fully open will permit a full-boreflow of water without any resistance. This valve has no washers and may be inclined to dribble very slightly, even

when fully closed. This dribble will not be so much that it will disallow basic maintenance work.

Check valves (F15) have a device within them that only allows water to flow one way, thus reducing the risk ofcontaminating the mains water.

Service valves (ball-fix valves) have a ball with a hole bored through it. When the ball is turned by means of a

lever or screwdriver, the water may be shut off. This allows for maintenance of taps, washing machines and so onwithout having to shut off and empty a complete system.

Service valves are available as straight (F16) and 90 degree bend (F17). These valves restrict the water flow unlike the ball valve, which is handle operated (F18) only require a quarter turn and do not restrict the flow ofwater. They are ideal for use in conjunction with showers.

F14

F15

F16

F17

F18


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HOW-TO 9

F20

Pressure relief valves (F20). If a closed system is overfilled with water or overheats, the pressure will rise.As a safety measure, this valve opens and allows water to escape until a safe pressure is reached.

Automatic air valves (F21) automatically purge air from the system. Air trapped in a system can stop water flowand cause radiators to remain cold.

Motorized valves (F22) are motor-driven and controlled either by the system programmer or a room thermostat.Most motorized valves have manual overrides for maintenance.

Lockshield valves (F23) control the flow of water through a radiator at the return end and are used to balancethe radiator within the system. After the plastic cap is removed, it is operated with a small spanner or screwdriver.

F21

F22

F23


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HOW-TO 10

F24

F25

F26

Hand-wheel valves (F24) turn a radiator on and off at the flow end.

Thermostatic radiator valves (F25) are used in some systems instead of room thermostats. The temperature ofeach radiator is controlled by a thermostatic valve, which can be set to close off at any given temperature.

Stop-valves are also called stop-cocks and turn water on and off (F27). They only allow water to flow one way.Some have a built in drain tap.

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How to understand central heating system